According to the recently released Climate Change Research Program report–CSSR—“ global average sea level has risen by about 7–8 inches since 1900. … Human-caused climate change has made a substantial contribution to this rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years. ….Global average sea levels are expected to continue to rise—by at least several inches in the next 15 years and by 1–4 feet by 2100. A rise of as much as 8 feet by 2100 cannot be ruled out. “
This assessment is consistent with the conclusion of the most recent IPCC report: “Over the period 1901 to 2010, global mean sea level rose by 0.19 [0.17 to 0.21] m. (7+inches) The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.”
These conclusions are both definitive and scary. They bring to mind pictures that have been published of the Washington Monument and New York City being seriously flooded. But such certitude goes well beyond what we know.
Raising a cautionary warning often results in being labeled a skeptic who is a tool of the fossil fuel industry. While that is a diversionary tactic, that reality makes it important to support caution with sources who are not viewed as skeptics and who are above reproach.
One such source is Professor Carl Wunsch, recognized as one of the world’s leading oceanographers. He is on the MIT and Harvard faculties In a briefing to EPA, he made a number of points that undermine expressions of certitude. “Sea level has been rising for about 16,000 years. In the last interglacial (period) it appears to have been a few meters higher than today.” According to Wunsch, sea level measurements have to be divided into two periods—before and after 1992 when satellite altimeters became the measurement technique of choice. Before 1992, attempts to determine average sea level changes relied on tide gauges and changes in temperature and salinity. But these attempts have been controversial because of the distribution of measurements, calibration of the devices, and interpretation of density changes.
While altimetry is the most accurate way to measure sea level changes, it is not without its own challenges. These include atmospheric factors, orbits, tides, and rotation wobble among others. Correcting these requires a high degree of accuracy to avoid interpretive errors.
Given the array of measurement challenges mentioned by Wunsch, he cautioned, “Global mean sea level is almost surely rising. Historical data are not adequate to compute accurate global averages. No mathematical trick compensates for missing data. Present multidecadal estimates of global averages have an element of fantasy about them.”
Last year, NOAA published a paper on sea level rise–NOAA—that contained a range for sea level rise in 2100 that went from a low of less than .5 meters to 2.5 meters. The importance of the NOAA estimate is that it demonstrates how complicated understanding of the ocean is and the uncertainties involved in predicting future sea levels. A major uncertainty is ice sheet melts. Perhaps the best example of that uncertainty is the variability that the IPCC puts around ice sheet melt in Antarctica. The error bars show that the ice sheet could grow as well as lose mass. In addition to melt, climate conditions like El Nino can cause a rise in levels at shorelines for months at a time and subsidence can also cause a higher sea level where it occurs
There are two lessons to be drawn that apply to more than sea level rise. First, projections about the distant future should be made with a great deal of humility. Second, when someone make a specific projection with great certitude, you can be certain that they are trying to persuade; not educate.